1. Field of the Invention
The present invention relates to an electrocatalyst having a good poisoning resistance to carbon monoxide, and also relates to an electrode and a membrane-electrode assembly each using said electrocatalyst, and further relates to a solid polymer electrolyte fuel cell using the same.
2. Description of the Prior Art
Since solid polymer electrolyte fuel cells have a high output power density and work at low temperatures and also scarcely evolve an exhaust gas containing harmful substances, they are expected as a propulsion power source for transportation means to replace internal combustion engines.
In the fuel cells a fuel gas such as hydrogen or methanol gas is fed to a fuel electrode (anode) and air or an oxygen-containing gas is fed to an oxidizer-electrode(cathode). The fuel is oxidized at the anode to produce protons and the oxygen is reduced at the cathode to form water, thereby generating electricity, as shown in the following formulas.
Anode reaction (in the case of hydrogen): EQU H.sub.2.fwdarw.2H.sup..fwdarw. +2e.sup.-
Cathode reaction: EQU 1/20.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O
Total reaction (in the case of hydrogen):
H.sub.2 +1/20.sub.2.fwdarw.H.sub.2 O
At the anode and cathode, electrocatalysts are used for accelerating the respective electrode reactions. As the electrocatalysts, there have hitherto been used platinum alone; a combination of platinum with at least one selected from palladium, rhodium. iridium, ruthenium, osmium and gold; a combination of platinum with at least one selected from base metals such as tin, tungsten, chromium, manganese, iron, cobalt, nickel, copper and the like. The electrocatalysts have been used in the form of powder of elementary metal or alloy which are optionally supported on conductive carbon particles.
Generally in the fuel cells (reformed gas fuel cells), there are used hydrogen-enriched gases which are obtained by previously reforming fuels such as alcohols and hydrocarbons by means of a reformer. However, in the electrodes of proton-conductive electrolyte fuel cells which work or operate at low temperatures, impurities such as carbon monoxide and carbon dioxide contained in fuel gasses poison platinum contained in the electrocatalyst to increase polarization and lower the output power of the cell. As a solution of this problem, it is reported to use platinum as an alloy of platinum with ruthenium, iridium, rhodium or the like (D. W. Mckee and A. J. Scarpellio Jr., J.Electrochem. Tech.,6(1969)p.101). It is also disclosed that a catalyst comprising a platinum-ruthenium alloy with an atomic ratio of the platinum to the ruthenium being about 1:1 supported on conductive carbon particles, has a high poisoning resistance (Japanese Pre-examination Patent Publication (kokai) Nos. 6-260207 and 9-35723).
Meanwhile, with regard to direct methanol fuel cells in which methanol is directly fed to the anode in generation of electricity, it is disclosed that a catalyst comprising platinum and ruthenium each in the form of an elementary metal, supported together on conductive carbon particles (WO97/21256), or a catalyst comprising platinum and ruthenium which are supported together on conductive carbon particles, respectively, in the form of an elementary metal and oxides (Japanese Pre-examination Patent Publication (kokai) No. 3-22361) show higher performance than the platinum-ruthenium alloy catalyst.
However, the conventional platinum-ruthenium binary anode catalyst is not sufficient in performance and therefore has been required to improve the performance. Especially, the alloy anode catalyst is low, as an anode catalyst used in reformed gas fuel cells, in poisoning resistance to carbon monoxide, so that it has a defect of a large anode-polarization. The alloy anode catalyst is also required, as an anode catalyst used in direct methanol fuel cells, to considerably lower the anode polarization.